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Is a much longer story but here is my take as preparations:
Q: Are you interested in the content only (take content databases) or also replicate farm and services configuration (security accounts, web applications, shared services, service pack level)? Are you planning a new server or existing server (go for X64 already if you can -gets you closer to SP2010)? depending on those questions you need to perform

Prepare your hardware, software updates and patches to reach at least the same level as the other farm (at least SP2 recomanded).

Once done you need to re-configure the new farm, install solution packages, and only at the end, transfer content. The less intrusive for your users is via COntent Database upgrade (copy across farms) and put them in Read-Only on the old system to avoid redo-ing it again due to meanwhile changes.

As I said, is more than that, but at least should get you started in reflection and planning